Use the Search module to add a plain search bar. Use the Layout Editor to add this module to a page.
The Search module shows results from your entire Confluence instance. If you prefer more formatting options, use a Search Highlight module.
Module settings
You have the following options:
Simple. Filter search results by label and space.
Advanced. Use CQL to create your own filter, for example:
Do a complete content search using the CQL: text ~"*$query*"
Do a complete content search on the space using the CQL: text ~"*$query*" and space.key=$spacekey